Site-specific recombination systems are important in controlling development and gene expression in a diverse array of organisms ranging from bacteria to humans. The long range goal of this project is to understand how bacteriophage X carries-out site-specific recombination. The findings will have a significant impact on other site-specific recombination systems because many of their integrases are related to the lambda Int protein. Biochemical and genetic approaches will be used to characterize the protein-protein and protein-DNA interactions that occur both during the assembly of recombination complexes (intasomes) and the process of strand-cleavage and exchange. The phage-encoded integrase (Int) protein is a central actor in intasome formation and strand exchange. Int mutants will be isolated and characterized in assays that will determine the defects of the individual proteins in the recombination pathway. The host-encoded integration host factor (IHF) also participates in intasome formation by inducing bends in the DNA. We will isolate mutants that have amino acid substitutions of residues that, as suggested by our previous genetic studies, interact with DNA. Such mutants will also provide information that will be useful in interpreting data derived from physical studies on the protein. The phage-encoded excisionase (Xis) and the host- encoded factor for inversion stimulation (FIS) promote excisive recombination. Xis interacts cooperatively with Int and FIS. We will characterize the mechanism(s) of cooperative protein-protein interactions by isolating and characterizing Xis mutants that are defective in interacting with Int or FIS.